In, for example, Near Field Communication (NFC) modules and wireless power transfer modules, the variability in electrical characteristics of a coil used for an antenna and the usage environment cause a predetermined resonant frequency to change. With decrease in size of mobile terminals in recent years, antennas also decrease in size. Therefore, the change in resonant frequency becomes unacceptable in circuit design. Thus, to adjust the resonant frequency, it has been known to use a variable capacitor of which the capacitance varies in accordance with a DC bias voltage as disclosed in, for example, Japanese Patent Application Publication No. 2010-55570.
There have been also known variable capacitors in which a plurality of capacitors each including a dielectric layer of which the permittivity varies in accordance with a DC bias voltage are connected in series between input/output terminals, and a variable voltage is applied to the plurality of capacitors as disclosed in for example, Japanese Patent Application Publication Nos. 2010-55570, 2011-119482, 2008-66682, 2005-64437, and 2014-103181. It has been known to make the resistance of a bias resistor coupled to two capacitors approximately ½ of the resistance of a bias resistor coupled to one capacitor as disclosed in, for example, Japanese Patent Application Publication No. 2014-103181.
It has been desired to make the response of the capacitance change to the change of the variable voltage faster. However, when the response is made to be faster, the characteristics including a Q-value and the like deteriorate.